1. Field of the Invention
The present invention relates to: a toner, for developing electrostatic latent images in electrophotography, electrostatic recording, and other processes; a production method thereof; and an electrostatic latent image developer using the same.
2. Description of the Related Art
Methods of visualizing image information via electrostatic latent images in the electrophotographic and other processes have been widely used in various applications. In these methods, visualization is realized by forming a latent electrostatic image on a photoreceptor (latent image bearing body) by charging/exposing in a electrophotographic process. This latent image is developed with an electrostatic latent image developer (hereinafter, referred to as “developer”) containing a toner for developing electrostatic latent images (hereinafter, referred to as “toner”), and transferred to and fixed on a recording medium. The developers used in these methods include: two-component developers, containing of a toner and a carrier; and one-component developers, containing only a magnetic or nonmagnetic toner.
Such toners are commonly produced in a kneading-pulverizing process, wherein a plastic resin is melt-kneaded with a pigment, an electrostatic charge-controlling agent, and a releasing agent (such as a wax) are then cooled, pulverized, and classified. Inorganic and organic particles are sometimes added to the surface of the toner particles to improve fluidity and cleaning property.
The recent move towards an information-society has driven a need for providing high quality images in documents by means. Hence intensive research has been conducted into improving the quality of images formed in various image forming processes. There is of course the same demand in the electrophotographic image forming process and, particularly in the electrophotographic process, there exists a need for a toner having a smaller diameter, and a narrower grain size distribution in order to produce images of higher definition.
However, with the kneading-pulverizing process commonly practiced in toner producer, there is a problem during pulverization and classification. A great amount of energy is required for the pulverization and this increases the cohesiveness of the toner particles, causing problems in the classification, particularly of particles. Thus the conventional process cannot satisfy the need for a reduction in the size of toner particles. In addition, the shape and the surface structure of such toner particles are irregular and, whilst slight variations can be made depending on the pulverization characteristics of the materials used and the conditions of the pulverization process, it is practically impossible to control the shape and surface structure of the intended toners deliberately.
Further, there is a restriction in selecting materials for use in the kneading-pulverizing process. More specifically, the resin/colorant dispersion should be brittle enough that the mixture can be pulverized into particles in economically feasible manufacturing equipment, However if the resin/colorant dispersion is brittle, the particles formed may be further pulverized into even finer particles by the mechanical shearing force applied in developing devices. As a result of these influences the following occur more readily: in the case of a two-component developer, the finer particles thus generated adhere to the surface of the carrier, accelerating charge degradation of the developer; while in the case of a one-component developer, the resulting expansion of the grain size distribution causes scattering of the toner and also changes in toner shape cause a deterioration in image quality due to the decrease in developing property of the toner.
When considerable amounts of a releasing agent such as a wax is added internally for production of a toner, the exposure of the releasing agent at the surface of the thermoplastic resin increases, depending on the combination thereof. In particular, use of a combination of a high molecular weight component resin which is high in elasticity, and thus less pulverable, together with a brittle wax, such as polyethylene or polypropylene, often results in increased exposure of the wax component on the surface of the toner. Such exposure is rather advantageous for the release during fixation and for cleaning of the untransferred toner from the photoreceptor. However the polyethylene on the surface is easily transferred by mechanical force onto the developing roll and the photoreceptor, making staining of the carrier more likely and reducing reliability.
In addition, such toners often do not flow sufficiently even with an addition of a flow-improving agent, since the toner shape is irregular, and so there is migration of the flow-improving agent into cavities on the toner surface due to the mechanical shearing force during use. This causes a decrease in fluidity over time, while the embedding of the flow-improving agent into the toner leads to a reduction in the developing, transfer, and cleaning properties of the toner. Further, reuse in the developing apparatus of the toner recovered in the cleaning unit often leads to a deterioration in image quality. Addition of a greater amount of the flow-improving agent to prevent of these problems causes staining, filming, blemishes, and the like on the surface of the photoreceptor.
Accordingly, various processes for producing toners different from the kneading-pulverizing process, employing various polymerization methods such as a suspension polymerization process and the like, have been examined [see e.g., Japanese Patent Application Laid-Open (JP-A) Nos. 60-57954, 62-73276, and 5-27476], and recently, a process for producing toners systematically by an emulsion polymerization aggregation method is proposed, as the means of controlling the shape and surface structure of toners (see e.g., JP-A No. 6-250439). Generally according to these methods toners are produced by: preparing a dispersion of resin particles by polymerization, for example, emulsion polymerization or the like; separately, preparing a colorant particle dispersion wherein a colorant is dispersed in a solvent; mixing these dispersions; aggregating the resin particles and the colorant particles together and growing the aggregated particles to a desired particle diameter by heating and/or pH adjustment, addition of a coagulant, or the like; then, stabilizing the aggregated particles at the desired particle diameter; and then, heating and coalescing the particles at a temperature of the glass transition point of the resin particles or higher.
The toner particles obtained in the emulsion polymerization aggregation process have extremely favorable properties (in particular, a narrower grain size distribution eliminating a need for classification), compared to those of the conventional toner particles obtained by the suspension polymerization process or other polymerization processes. The use of these particles as a toner allows the formation of high quality images over an extended period of time. In addition, the toner production process by the emulsion polymerization aggregation method, wherein the aggregated particles are heated and coalesced at a temperature of the glass transition point (Tg) of the resin particles or more, allows production of toners of a variety of different shapes from amorphous to spherical, by proper choice of the heating method and proper pH adjustment. Accordingly, it becomes possible to select the shape of the toners tailored to the specific electrophotographic system used, in the range from so-called potato-shaped to spherical.
On the other hand, when considering the reliable reproducibility of electrostatic latent images small diameter spherical toners with weaker adhering forces and superior developing and transfer properties have been favored. But when used in a relatively inexpensive blade-cleaning system wherein the toner remaining after transfer on the latent image bearing body is cleaned by a blade, these smaller spherical toners are inferior in cleaning, often causing problems such as black lines, colored lines, and the like due to improper cleaning. Amorphous toners are superior in cleaning with in the blade-cleaning system, but the transfer and developing properties gradually decrease because of the migration of the external additives into the cavities of toners, and local embedding of the external additives in the toners due to the stress in the developing device. This leads to problems such as: deterioration in image quality; generation of fogging of the substrate; increase in the amount of toner consumed, due to decrease in transfer efficiency; and the like.
For the reason above, potato-shaped toners (shape factor SF1 (described below): 125 to 140) are widely used in the electrophotographic systems employing the relatively inexpensive blade-cleaning system. However, from the viewpoint of particle shape, the potato-shape particles have a wide shape distribution, and, as it is impossible to control each of the shape and the uniformity of surface of the toner separately. The particles hence have wider ranges of distribution in shape and in the degree of uniformity of surface. The potato-shaped particles contain both incompletely coalesced particles, having irregular surfaces, and completely coalesced particles having a smooth surface. Even in the emulsion polymerization aggregation process wherein the diameter and the shape of toner particles are controllable more easily than in other production processes, it is very difficult to control the surface properties of toners at will. Also because only toners in a very narrow region of shapes can satisfy all of the requirements for developing, transfer, and cleaning properties, very exact control of the production conditions is required.
Considering recent demands for higher speeds and lower energy consuming devices, toners having uniform electrostatic propensity, durability, higher toner strength, and narrower grain size distribution are becoming more and more important. Also the need to improve speed whilst reducing the energy consumption of these devises indicates that it is necessary to fix images at even lower temperatures. A releasing agent component is added to the toner for the purpose of improving the image fixing properties, and a polyolefin-based wax is commonly added internally as the releasing agent component for prevention of low-temperature offsetting during fixing. In addition, a small amount of silicone oil is applied uniformly on the fixing roller for improvement in high-temperature offset ability. As a result, the silicone oil components are adhered to the surface of the output recording body, making it sticky, or the like, and unpleasant to handle.
To solve the problem, an oil-less fixing toner, containing a great amount of a releasing agent component, is proposed (see e.g., JP-A No. 5-61239). However, although the addition of a large amount of releasing agent is effective to some extent in improving the high-temperature offset ability, the binder resin component and the releasing agent are mutually compatible, prohibiting consistent and uniform release of the releasing agent and thus stability in high-temperature offset resistance is not easily obtained. Because the cohesiveness of the binder resin in the toner is governed by the weight-average molecular weight (Mw) and Tg of the binder resin, it is difficult to control the internal and surface structures of the releasing agent wax at the same time, and thus it is practically impossible to control directly the stringiness, cohesiveness, and high-temperature offset ability of the toner during fixing. Further, liberated components from the releasing agent may sometimes cause inhibition of charging.
To overcome these problems, some methods of compensating for the rigidity of binder resin by an addition of high-molecular weight component or the introduction of chemical crosslinking is proposed. This has the effect of reducing the stringiness of toner at the fixing temperature and improves the high-temperature offset ability in the oil-less fixer (see e.g., JP-A Nos. 4-69666, 9-258481, 59-218459, and 59-218460). However, when simply a cross-linking agent component is added to the binder resin, the viscosity of toner, i.e., the cohesive forces in the molten state, increases and the rigidity of the binder resin increases. Whilst the temperature related dependency, toner load related dependency, and the like of oil-less fixing may be improved to some extent, as a result of the increased rigidity flexural resistance to bending of fixed images declines. It becomes practically impossible to control together both the temperature and the toner load related dependencies of peeling. In particular, when used in an energy-saving type fixing device processing at low temperature and low pressure, or an copying machine or printer having a higher printing speed, such toners cannot really provide satisfactory fixed images.
As described above, currently, there are no toners produced in any one production processes, including the kneading-pulverizing process, suspension polymerization process, and emulsion polymerization aggregation process, which can satisfy all the requirements for fixability, image quality, developing consistency and developing, transferring and cleaning properties.